1. Field of the Invention
The present invention relates to an optical recording medium capable of rewriting data, and more particularly to such a medium for use in an external storage device of a computer, a video and/or audio recording device, a storage device of a game machine and the like, or multi-media combining these devices.
2. Description of the Prior Art
In recent years, a magneto-optical recording medium which is one of optical recording media capable of rewriting data thereon has been put into practical use as an optical recording medium capable of reading, writing and erasing data.
The following describes an exemplified conventional magneto-optical recording medium with reference to the attached drawings.
FIG. 4 shows a construction of a conventional magneto-optical recording medium. Referring to FIG. 4, the magneto-optical recording medium comprises a plastic substrate 1 provided with a tracking guide groove for tracking servo, a dielectric film 2 having a high refractive index, a magneto-optical thin film 3, an intermediate layer 4, a reflection film 5, and a protection layer 6.
The substrate 1 is usually composed of a polycarbonate substrate because the material can be easily molded and produced at a low cost. The dielectric film 2 having a high refractive index is composed of any of various dielectric materials such as silicon nitride, aluminum nitride, zinc sulfide, selenium sulfide, tantalum oxide, or a mixture of any of the materials and silicon oxide. Assuming that the refractive index is n and the wavelength of laser beam in the data read/write time is .lambda., the dielectric film 2 is made to have a film thickness of .lambda./4n or less. Generally, silicon nitride, aluminum nitride, or tantalum oxide having a refractive index of 2.0 is used as the dielectric film 2 at a laser wavelength of 780 or 830 nm for read/write/erase of data, and therefore the dielectric film 2 having a high refractive index of 2.0 is usually made to have a film thickness of 100 to 110 nm.
The magneto-optical thin film 3 is usually made of an alloy of a rare earth such as Gd, Tb, or Dy and a transition metal such as Fe or Co, which is made to have a Curie point of 170.degree. to 210.degree. C. and made to have a film thickness of 20 to 40 nm.
The intermediate layer 4 is composed of any of various dielectric materials such as silicon nitride, aluminum nitride, zinc sulfide, selenium sulfide, tantalum oxide, or a mixture of any of the materials and silicon oxide in the same manner as the dielectric film 2, and made to have a film thickness of 10 to 40 nm.
The reflection film 5 is made of a metal mainly composed of aluminum and made to have a film thickness of 30 to 60 nm. The magneto-optical thin film 3 and the reflection film 5 are further protected by the protection layer 6 which is formed by laminating a polymer layer or laminating polymer and dielectric layers.
The above-mentioned conventional construction has caused no trouble at the wavelength (780 nm or 830 nm) of laser beam. However, in order to execute read/write/erase of data at a shorter wavelength (680 nm, 532 nm, or less) for the purpose of achieving the more high-density recording, a new problem arises. The problem is that, since the film thickness of the dielectric film 2 is set at around .lambda./4n, the film thickness is required to be thinner according as the wavelength is reduced in proportion to the wavelength .lambda. of laser beam in executing read/write/erase of data.
The dielectric film 2 has two functional effects, one is an effect of enhancing a rotation angle of linear polarization plane obtained by the magneto-optical thin film 3. The other is an effect of thermally insulating between the plastic substrate 1 having a low heat resistance (low softening temperature) and the magneto-optical thin film 3.
In the data erasing time, generally a greater power is used to securely erase the recorded marks (written domain), and therefore the temperature of the magneto-optical thin film 3 is momentarily increased up to a temperature of about 400.degree. C. In regard to the above, the reason why the substrate of any conventional magneto-optical recording medium does not deteriorate despite the fact that a plastic substrate made of polycarbonate resin has a softening temperature of 130.degree. to 140.degree. C. is that the dielectric film 2 has a heat insulation effect concurrently with the enhancement effect.
However, when the dielectric film 2 is made thinner in proportion to .lambda./4n as the wavelength of laser beam is reduced, the heat insulation effect gradually degrades. Particularly when data write/erase operations are repetitively executed many times, the substrate suffers a thermal damage. The above-mentioned problems are still more serious for the zinc sulfide, selenium sulfide having a high refractive index of 2.2 to 2.3, or a mixture of either of the materials and silicon oxide. This is because, in the construction, the dielectric film 2 made of the above-mentioned materials has a high refractive index and a film thickness at which the enhancement effect is maximized as described above, the film thickness is required to be thinner because of the high refractive index of the film, and therefore the optimum film thickness is reduced to about 80 nm even at the wavelength of laser beam of about 780 nm.
In the case where the dielectric film 2 is composed of a film made of a material such as silicon nitride, aluminum nitride, tantalum oxide having a refractive index of about 2.0, the film thickness is not greater than 90 nm according to a film thickness setting at which the enhancement effect is maximized when in read/write operation at a short wavelength .lambda. of laser beam of 680 nm or less. In the above case, the substrate suffers a thermal damage.
The following describes in detail the thermal damage of a plastic substrate, which has never been obvious before.
When more than several tens of tracks are subjected to continuous data erase/write operations, the groove provided on the substrate gradually deforms due to the repetitive operations. Furthermore, the reflectance at land portions between grooves is slightly reduced, and primary diffraction light increases to result in the increase of the push-pull signal for the tracking. In other words, the groove is gradually deformed. In the above case, when scores of tracks are subjected to continuous data erase/write operations, a temperature gradient takes place in the radial direction of the data recording medium concurrently with heat accumulation in the substrate. Consequently, a deformation of the groove takes place unbalancedly between the inner slant surface and the outer slant surface of the groove to be a cause of tracking offset. This problem is caused by heat accumulation in the substrate at the time of continuous erase/write operations to the tracks, which becomes more serious as the track pitch is made narrower.
The quantity of tracking offset itself is small to cause less trouble. However, when data erase/write operations are effected on more than several tens of continuous tracks with, for instance, a data regenerating laser power applied to the header section and a data erasing laser power applied only to a data section in the data erasing time, the header section does not change at all, whereas tracking offset occurs only in the data section. Particularly at the point of change between the header section and the data section, radial acceleration in the tracking direction increases to exert a significantly bad influence on the tracking servo.
FIG. 5 shows tracking error signal waveforms obtained in the data regenerating time by effecting a data erasing operation on continuous 300 tracks of a magneto-optical recording medium in which the dielectric film 2 has a film thickness of 80 nm at a wavelength of 780 nm of erasing laser beam and repeating operation many times. The recording medium used has a spiral groove directed from an inner periphery to an outer periphery having a track pitch of 1.6 .mu.m, and a data erasing operation was effected on 300 tracks continuing from the inner periphery to the outer periphery.
Referring to FIG. 5, A1 denotes a tracking error signal waveform obtained in an initial data reading stage, A2 represents a tracking error signal waveform obtained in the data reading stage after one time of data erasing operation, A3 represents a tracking error signal waveform obtained in the data reading stage after 10 times of data erasing operations, A4 represents a tracking error signal waveform obtained in the data reading stage after 100 times of data erasing operations, A5 represents a tracking error signal waveform obtained in the data reading stage after 1,000 times of data erasing operations, A6 represents a tracking error signal obtained in the data reading stage after 10,000 times of data erasing operations, and A7 represents a laser drive signal waveform obtained in the data erasing stage where the high level is the normal data erasing power of 8 mW and the low level is the normal data reading power of 1.5 mW.
As is evident from FIG. 5, the tracking is deviated inward at the time of turning on the data erasing power, and the tracking is deviated outward at the time of turning off the data erasing power, for which reason the tracking servo control is unstable. The above is because more severe deformation takes place on the inner slant surface then the outer slant surface of the groove when the data erasing operation is repetitively effected on continuous 300 tracks more than 1,000 times, and therefore an inward tracking offset occurs at the portion to which the high data erasing laser power is applied.
The above-mentioned problems occurs because the film thickness of the dielectric film 2 having a heat insulation effect between the substrate and the magneto-optical film is set at around .lambda./4n and the film thickness is reduced when the film comprises zinc sulfide, selenium sulfide, or the like having a high refractive index as a main ingredient, which results in reducing the heat insulation effect between the substrate and the magneto-optical film.